For downhole power generation, various methods of transmitting power from the surface to devices within the borehole have been proposed, such as placing cables in an annulus formed between the borehole and casing placed within the borehole, or use of electrical conduction through the casing. The use of sea water batteries, being isolated cells which are placed on the sea floor to provide local power generation for sub-sea production systems, is also known. However the use of batteries within boreholes is a problem as there is only space for small batteries within boreholes, and these batteries have a current generating lifetime of only a few days, due to the high temperatures downhole.
It is one aim of the present invention to provide downhole power generation method and apparatus with an extended life time for generating electricity.